1. Field of the Invention
The present invention relates to a printing apparatus having a system for stirring pigment ink, and a pigment ink stirring method.
2. Description of the Related Art
When an inkjet printing apparatus using pigment ink as a printing material has not printed for a long time and the pigment ink is left to stand for a long time, the pigment concentration (space distribution) of the pigment ink becomes nonuniform in the ink tank due to gravitational sedimentation of pigment particles heavier in specific gravity than the solvent of the pigment ink. A printing operation after long-term inexecution of printing cannot print with optimum image quality. To prevent this, an inkjet printing apparatus (to be referred to as a printing apparatus) in which an ink tank is mounted on a carriage horizontally moves the carriage to violently shake the ink tank, stirring the pigment ink in the ink tank. This homogenizes the pigment particle concentration (space distribution) in the ink tank.
For example, Japanese Patent Laid-Open No. 2007-301772 discloses a method of setting a carriage reciprocal movement condition to stir ink in an ink tank in accordance with the printing amount in a printing operation if the elapsed time after the end of reciprocal movement of the carriage on which a printhead and ink tank are mounted is shorter than a predetermined time. By reciprocating the carriage in accordance with the reciprocal movement condition, ink can be efficiently stirred depending on the ink status in the ink tank. As a result, ink such as pigment ink in the ink tank can be homogenized, printing a high-quality image.
Japanese Patent Laid-Open No. 8-258281 discloses an inkjet head including an ink tank which contains ink, an ink filter connected to it, a common ink chamber connected to the filter, and a plurality of nozzles connected to the common ink chamber. The head further includes ink orifices corresponding to the respective nozzles, at least a pair of electrodes which are arranged in the ink tank and have different surface areas, and an AC voltage application unit which applies a voltage to these electrodes different in surface area. With this arrangement, when an AC voltage is applied to the pair of electrodes different in surface area, the electrode smaller in surface area can absorb dust and dirt in ink. This can implement an ink discharge device which removes, from ink, dust and dirt large enough to cause clogging and prevents clogging of the ink filter even when the mesh of the ink filter is set to a size small enough to remove small dust and dirt.
In an inkjet printing apparatus using pigment ink, to efficiently stir and homogenize pigment ink whose pigment particles gravitationally sediment, an arrangement which reciprocates the ink tank-mounted carriage is effective, as disclosed in Japanese Patent Laid-Open No. 2007-301772.
However, when the printhead and ink tank are separated and only the printhead is mounted on the carriage to increase the ink tank capacity and lighten the carriage, the arrangement disclosed in Japanese Patent Laid-Open No. 2007-301772 cannot stir pigment ink. This is because the ink tank is fixed to the printing apparatus main body and even moving the carriage cannot stir pigment ink in the ink tank. Hence, another stirring method is necessary for the arrangement in which the ink tank is fixed to the printing apparatus.
Further, the arrangement disclosed in Japanese Patent Laid-Open No. 2007-301772 does not include a means for detecting whether or not pigment ink has been stirred. The stirring count is set based on the elapsed time after a previous carriage reciprocal operation and the print data amount. However, in this arrangement, if the ink tank is replaced after a previous carriage reciprocal operation, the stirring operation may become insufficient or take time more than necessary.
In the arrangement disclosed in Japanese Patent Laid-Open No. 8-258281, the electrode smaller in surface area can absorb pigment particles, similar to dust and dirt. However, pigment ink in the ink tank cannot be homogenized. Thus, a means for causing convection in pigment particles to positively stir the pigment ink is required.